1. Field of the Invention
This invention relates to analog electric circuits, and more particularly to switching circuits in which the switch element is implemented with a metal-semiconductor field effect transistor (MESFET).
2. Description of the Prior Art
The JFET has had a dramatic effect on the development of modern monolithic operational amplifiers and switch circuits. The device was first proposed by Shockley in 1952. It uses the depletion region of a reverse-biased PN junction to modulate and "pinch off" the cross-sectional area of a channel region between the drain and source by varying the thickness or depth of the channel in which the drain current flows. The channel is of the same type doping as the source and drain regions, and is created during the fabrication process. Because the channel exists and conducts current when the gate-source voltage is zero, JFETs are generally referred to as depletion mode devices; the application of an appropriate gate-source voltage will typically deplete the channel and thereby reduce the drain current flow. However, enhancement-mode MESFETs have also been fabricated.
A relatively new JFET structure, the metal-semiconductor FET (MESFET) is a very high frequency device that can provide gain at microwave frequencies. It uses a Schottky contact for the gate on an n-channel JFET. A thin layer of N-type gallium arsenide (GaAs) is used for the channel region. The spread of the depletion region of the Schottky-gate contact into the N-type region of the GaAs modulates the drain-source current flow. The structure can be made very small, and no diffusions are needed in the fabrication. The GaAs substrate is a better insulator than silicon and makes a good supporting structure.
One difficulty encountered in using a JFET as an analog switch is that the gate-source must not be forward biased by more than about 0.5 volts. Otherwise gate current will flow into the transistor, which can cause the output voltage to vary from its desired matching with the input. This contrasts with a metal-oxide semiconductor FET (MOSFET), which can be switched by simply raising its gate voltage up to a positive level in excess of threshold to turn it on, or keeping its gate voltage at zero or a negative level to hold it off.
Switching circuits for silicon JFETs have been devised which use bipolar transistors to drive the switching JFETs. Such a circuit is utilized, for example, in the SW-01/SW-02 JFET analog switches produced by Precision Monolithics Inc., the assignee of the present invention, and illustrated in the Precision Monolithics Inc. 1988 Analog IC Data Book, pages 13-8 through 13-14. The MESFET fabrication process, however, does not lend itself to combinations with bipolar transistors.
A simplified schematic of a JFET analog switching circuit that may be said to represent the state of the art prior to the present invention is shown in FIG. 1. In this circuit, a switching JFET 2 has its drain connected to the input terminal 4 for an input analog signal, and its source connected to an output terminal 6. The drain is connected to the device's gate through a resistor 8 which limits current variations due to processing differences. The switch is operated by a digital control signal applied to control terminal 10 at the gate of enhancement-mode JFET 12, the drain of which receives current from the gate of JFET 2 through Schottky diode 14, and the source of which is connected to a negative voltage terminal 16. While this circuit does switch an analog input voltage in response to a signal at the control terminal, this operation is degraded by a current flow from the input terminal 4 down to the negative voltage terminal 16 when the switch is off.